The present invention relates to a control rod for a nuclear reactor, and more particularly, to a control rod for a nuclear reactor of the long-life type having an improved mechanical soundness in a boiling water reactor.
A control rod for a boiling water reactor (BWR) has usually four wings formed by housing neutron absorber plates in a plurality of long sheaths having a deep U-shaped cross-section. A leading end structural member is provided at an insertion leading end portion of each of the wings, or a terminal end structural member, at an insertion terminal end portion thereof, and the U-shaped openings of the sheaths in each of four wings are secured to an integral type center structural member (known also as a tie rod) having a cruciform cross-section to provide a cruciform sectional configuration or arrangement.
In a conventional control rod, the sheath is made of stainless steel (S.S such as SUS, hereinafter abbreviated as "SUS"), and a SUS tube having a diameter of 5 mm filled with boron carbide (B.sub.4 C) powder has been employed as a neutron absorbing rod.
Boron (B) has however a short nuclear life because it reacts with neutron to generate helium (He) and lithium (Li), resulting in a degraded neutron absorbing ability, and helium causes an increase in internal pressure, leading to a decrease in soundness of the SUS tube and hence to a shorter mechanical and physical service life.
In order to provide a control rod having a long service life, there has been used a long-life type control rod manufactured by replacing a conventional neutron absorbing rod partially or totally by hafnium (hereinafter abbreviated as "Hf") which is a long-life type neutron absorber.
Since Hf has a large specific gravity (density) as about 13, an Hf rod having the same cross-section as a conventional neutron absorbing rod using boron carbide results in a weight about 1.5 times as large as the control rod as a whole, although the neutron absorbing ability (reactivity value) is substantially the same, making it impossible to back-fit it into a nuclear reactor in operation.
As a counter-measure, Japanese Patent Laid-open (KOKAI) Publication No. HEI 1-34358 "Control Rod for Nuclear Reactor" proposes an Hf control rod of the type known as a trap type in which Hf is formed into a plate shape, and two Hf plates are arranged opposite to each other with a gap for introduction of water.
Further, in view of the fact that in about the terminal end side half of a control rod for the BWR, upon insertion thereof into the reactor core, a decreased neutron absorbing ability would cause no inconvenience in the control of the BWR, a control rod configured so as to use a smaller Hf content in the portion on the insertion terminal end side than in the portion on the insertion leading end side is proposed in Japanese Patent Laid-open Publication No. HEI 7-3468 "Control Rod for Nuclear Reactor".
Regarding the long-life control rod having the trap structure using the Hf plate, excellent results have already been achieved in many BWRs, and it is the usual practice to set a short service life for maintenance purposes.
When setting a longer service life, it becomes now clearer than ever that it is effective to improve mechanical strength of SUS structural members such as a sheath in the control rod.
FIGS. 19 to 21 illustrate an outline of an Hf trap type control rod, in which FIG. 19A is a partially cutaway perspective view, FIG. 19B is a sectional view of a wing and FIG. 19C is a perspective view of a load supporting member (also referred to as a "load supporting spacer" of "top spacer").
FIG. 20A is a partially cutaway front view of a sheath shown in FIG. 19A, and FIG. 20B illustrates an example of thickness of an Hf plate which is a neutron absorber plate of a neutron absorbing material attached in the interior of a sheath, as illustrated in a distribution diagram in the control rod insertion/withdrawal direction which is the sheath longitudinal direction.
FIG. 21A is a partially enlarged front view of FIG. 20A, FIG. 21B is an enlarged front view of a pair of Hf plates shown in FIG. 21A, and FIG. 21C is a sectional view of FIG. 21C taken along the line XXIC--XXIC of FIG. 21B.
Referring to these figures, a long-life type control rod 1 has a cruciform section with four wings 2, and a leading end structural member 4 integral with a handle 3 is secured to the insertion leading end portion into the reactor core, and a terminal end structural member 5 is secured to an insertion terminal end portion.
Further, a cruciform integral type center structural member 6 made of SUS is provided at an axial center of the control rod 1 (central tie rod), and an opening portion of a sheath 7 made of SUS having a deep U-shaped cross-section forming an outer periphery of the wing 2 is secured by welding to each projection of this integral type center structural member 6.
A plurality of sheath holes 8 and water holes 9 are pierced in the sheath 7, in which two Hf plates 10 which are neutron absorber plates are supported by a load supporting member 12 also serving as a gap (interval) maintaining space, and a water gap 11 (gap through which cooling water flows during use in the reactor) is formed between the two Hf plates 10.
The load supporting member 12 has a top-like shape, and the thickness of a gap maintaining portion 12a at the center thereof has a function of spacer. The Hf plates 10 are supported by attaching the Hf plates 10 from both the sides to a support shaft 12b through an attachment hole 13 and causing engagement of the support shaft 12b with a sheath hole (bore) 8, which are secured together by means of welding.
When inserting or withdrawing the control rod 1 into or from the reactor core, a percussive force is applied to the sheath 7 upon intermittent driving, or particularly, upon starting driving or decelerating during scram of the reactor.
In a long-life type control rod 1, the sheath 7 and the load supporting member 12 made of SUS forming the wing 2 have a thermal expansion coefficient about three times as high as that of the Hf plate 10, which is the neutron absorber formed of a material different from those of the sheath 7 and the load supporting member 12. For example, while SUS has a thermal expansion coefficient of 17.8.times.10.sup.-6 /deg-C., that of Hf is 5.9.times.10.sup.-6 /deg-C. ("Nuclear Reactor Materials Handbook" published by Nikkan Kogyo Shinbun-sha).
To avoid inconveniences resulting therefrom, the attachment hole 13 of the Hf plate 10 to be attached to the support shaft 12b of the load supporting member 12 has a diameter larger than that of the support shaft 12b to provide a margin, thereby permitting avoidance of their mutual interference through expansion and contraction in heat cycles during operation of the reactor.
In the example shown in FIG. 20, the Hf plate 10 of the control rod 1 having a length L in the inserting direction into the reactor core or the sheath longitudinal direction is longitudinally and equally divided into eight sections. The length 1 of a single Hf plate 10 is therefore about L/8.
FIGS. 20 and 21A are described with a scale compressed in the axial direction for convenience of illustration, and FIG. 21B represents the Hf plate substantially similarly to the actual one.
Within the wing 2, two Hf plates 10 which are neutron absorber plates arranged opposite to each other form an Hf plate pair 14 which is held by the sheath 7 through four (or three, five or six) load supporting members 12.
The attachment hole 13 of the Hf plate and the sheath hole 8 of the sheath 7 have the same pitch 15 size in the sheath longitudinal direction.
During inserting or withdrawing operation of the control rod 1, the sheath 7 is subjected, not only to a static load caused by the weight of the Hf plate pair 14 applied through the load supporting member 12 in the stationary state, but also to a dynamic load caused by the relative displacement to the Hf plate pair 14. This load caused by relative displacement becomes an impact load particularly upon intermittent operation or driving starting operation or decelerating operation during a rapid driving in the scram of the reactor.
These loads tend to be believed to be shared by the four load supporting members 12 in general and transmitted to the sheath 7. Actually, however, even when a margin is provided taking account of a difference in thermal expansion caused by the difference in thermal expansion coefficient between different materials such as between the attachment hole 13 of the Hf plate and the support shaft 12b of the load supporting member 12, it is conceivable that a single load supporting member bears all the load because of, for example, the manufacturing tolerance.
In the worst case, a specific unknown load supporting member 12 receives a large stress, thus causing a local stress to concentrate on the sheath hole 8 at the position where that load supporting member is secured. This is therefore undesirable from the point of view of ensuring soundness of the sheath 7.
In the Hf plate 10 of the Hf plate pair 14, as shown in FIG. 20B, the neutron irradiation is larger at a position closer to the insertion leading end and the reactivity value must be increased. The thickness becomes therefore thicker toward the insertion leading end and thinner toward the insertion terminal end.
The length in the control rod insertion/withdrawal direction, which is the sheath longitudinal direction of each Hf plate 10, is usually constant, and the thickness of the sheath 7 is also uniform in the control rod insertion/withdrawal direction. A wider water gap corresponds to a higher reactivity value.
The sheath 7 should therefore be preferably the thinnest possible. However, since the thickness of the sheath 7 is associated with strength of the sheath 7, an excessive reduction of thickness causes deterioration of mechanical soundness, thus preventing improvement of service life of the control rod 1.
More specifically, the distribution in the sheath longitudinal direction of the load acting on the sheath 7 is such that the load is larger toward the insertion leading end because of the thicker Hf plate 10. When designing the thickness of the sheath 7, therefore, it is necessary to sufficiently take account of the weight of the Hf plates 10 serving as the neutron absorber plates and the impact load received upon operation of the control rod 1, as well as mechanical strength with due regards to the manufacturing tolerance.
When a horizontal impact is caused by an earthquake or the like, a relatively large stress occurs near the central portion in the longitudinal direction of a long control rod 1. Ensuring a satisfactory mechanical strength for the proximity of the middle is an important task.
Therefore, using the control rod 1 for the longest possible period of time contributes to improvement of reliability of the control rod and economic merits of reactor operation. In order to further extend the service life of the long-life type control rod 1, it is necessary to increase mechanical strength having so far formed a restriction as compared with the nuclear life in neutron absorption.